Foundations in Microbiology

Chapter6

PowerPoint to accompany

Fifth Edition

Talaro

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An Introduction to the Viruses

Chapter 6

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Size of viruses

Naming viruses

• No taxa above Family (no kingdom, phylum, etc)

• 19 families of animal viruses• Family name ends in -viridae ,

Herpesviridae• Genus name ends in -virus, Simplexvirus• Herpes simplex virus I (HSV-I)

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• Family – Herpesviridae• Genus – Varicellovirus• Common name – chickenpox virus• Disease - chickenpox

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capsids

• All viruses have capsids- protein coats that enclose & protect their nucleic acid

• Each capsid is constructed from identical subunits called capsomers made of protein

• 2 types: – helical – iscosahedral

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helical

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icosahedral

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icosahedral

• 20-sided with 12 corners• Vary in the number of

capsomers• Each capsomer may be

made of 1 or several proteins

• Some are enveloped

complex

Fig 6.9a,c

6 steps in phage replication1. adsorption – binding of virus to specific

molecule on host cell2. penetration –genome enters host cell3. replication – viral components produced4. assembly - viral components assembled5. maturation – completion of viral formation6. release – viruses leave cell to infect other cells

13Fig 6.11

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penetration

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Bacteriophage assembly line

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• Not all bacteriophages lyse cells• Temperate phages insert their viral DNA

into the host chromosome & viral replication stops at there until some later time.

• Lysogeny- bacterial chromosome carries phage DNA

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Host range

• Spectrum of cells a virus can infect – cell has to have a specific structure (receptor) on its

surface for viral attachment– cell has to contain all of the enzymes and materials

needed to produce new virions• May be one species or many

– HIV (only humans) vs rabies (many animals)• May be one tissue or many within a host

– Hepatitis (liver) vs polio (intestinal & nerve cells)

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Differences between phage and animal virus replication

1. Animal virus replication is more complex than phage replication because host cells are more complex.

2. Animal viruses cannot inject their DNA.3. Lysogeny for phage, latency for animal

viruses

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Animal virus replication

1. adsorption2. penetration/uncoating of genome3. duplication/synthesis4. assembly5. release

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adsorption

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penetration

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Release by budding

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Cytopathic effects- virus-induced damage to cells

1. changes in size & shape2. cytoplasmic inclusion bodies3. nuclear inclusion bodies4. cells fuse to form multinucleated cells5. cell lysis6. alter DNA7. transform cells into cancerous cells

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Cytopathic changes in cells

How do we grow viruses?

Obligate intracellular parasites require appropriate cells to replicate.

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Growing animal viruses

1. live animals 2. bird embryos – chicken, duck; intact, self-

supporting unit, sterile, self-nourished3. cell culture

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Other noncellular infectious agents1. prions - misfolded proteins, contain no nucleic acid

– cause spongiform encephalopathies – holes in the brain– common in animals

• scrapie in sheep & goats• bovine spongiform encephalopathies (BSE), aka mad cow disease• humans – Creutzfeldt-Jakob Disease

2. viroids - short pieces of RNA, no protein coat– only been identified in plants, so far

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Diagnosis of viral diseases

• More difficult than other agents• Consider overall clinical picture• Take appropriate sample

– Infect cell culture- look for characteristic cytopathic effects

– Screen for parts of the virus– Screen for immune response to virus

(antibodies)

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diagnosis